Adaptive echo elimination and adaptive elimination of noise caused by intersymbol interference are usually resorted to in telephony and data communication technology for improving transmission quality. Echo noise comes from the signal of the local transmitter and occurs as leakage signals in hybrid circuits as well as echoes at the remote end and in non-homogenous lines. Noise caused by intersymbol interference comes from the signal from the remote end and occur particularly at transmission on non-homogenous lines.
To reduce the effect of these two kinds of noise, it is already known to utilize a balance filter and an equalizing filter of the digital type. With guidance from a given number of digital data symbols sent from the local transmitter, the balance filter forms an estimation of the expected value of the echo, and with guidance from a given number of the latest received digital data symbols the equalizing filter forms an estimate of the expected value of the intersymbol interference. The estimates formed by the filters are subtracted from the received signal, the difference signal being subsequently utilized for determining the value of the received data symbols and for correcting the filter parameters. These parameters are adjusted automatically so that the deviations between the true and the estimated echo and between the true and the estimated intersymbol interference will be as small as possible. After the adjustment of the parameters, they are not altered at all or only insignificantly. It is then usual to say that the filters have converged.
In certain cases the convergence time for a balance filter or an equalizing filter will be of comparatively long duration or it may even be impossible to achieve convergence. An advantageous convergence method solely for a balance filter is described in Swedish patent application No. 8,106,444-6.
In simultaneous adaptive echo elimination and adaptive elimination of noise which is caused by intersymbol interference, there are, however, special problems with adjusting the filter parameters, due to the residue echo signal's usually being much greater than the signal from the remote end before the filters have converged. This causes the equalizing filter to adjust itself in principle as a further balance filter, with the consequence that convergence will be slow and uncertain. The problem is discussed in an article published in "I.E.E.E. JOURNAL ON SELECTED AREAS IN COMMUNICATIONS", VOL. SAC-2, NO. 2, MARCH 1984, pp. 314-323.
In "THE BELL SYSTEM TECHNICAL JOURNAL", VOL. 58, No. 2, FEBRUARY 1979, pp. 491-500, the adjustment of the filter parameters is analyzed with the condition that the received data is perfectly regenerated, which is very unlikely, according to the above.